Stimulating and surging an earth formation

ABSTRACT

A method of enhancing communication between a wellbore and an earth formation intersected by the wellbore can include forming a fracture in the formation, and permitting communication between the wellbore and a low pressure volume in response to a desired characteristic of the fracture being maximized. Another method of enhancing communication between a wellbore and an earth formation can include forming a fracture in the formation, and then, while pressure in the formation proximate the wellbore is at least about a closure pressure of the fracture, decreasing pressure in the wellbore by exposing the wellbore to a low pressure volume.

BACKGROUND

This disclosure relates generally to equipment utilized and operations performed in conjunction with a subterranean well and, in an example described below, more particularly provides for stimulating and surging an earth formation.

It is increasingly important to provide economical and efficient ways to exploit resources, so that the resources are not wasted. One way of achieving this goal is to provide for enhanced fluid communication between wellbores and formations penetrated by those wellbores.

Stimulating formations and reducing restrictions to flow between wellbores and formations can promote such enhanced fluid communication. Therefore, it will be appreciated that improvements are needed in this art.

SUMMARY

In the disclosure below, systems and methods are provided which bring improvements to the art of improving fluid communication between a formation and a wellbore. One example is described below in which only a single trip of a perforating string into a wellbore can accomplish perforating, stimulating and surging of a formation. Another example is described below in which surging of the formation occurs while increased pressure is applied to the formation.

In one aspect, this disclosure provides to the art a method of enhancing communication between a wellbore and an earth formation intersected by the wellbore. The method can be performed by fracturing the formation in response to firing a perforator in the wellbore, and then, a predetermined delay period after firing the perforator, decreasing pressure in the wellbore. In some examples, pressure in the wellbore may be decreased to less than formation pressure.

In another aspect, this disclosure describes an example of a method of enhancing communication between a wellbore and an earth formation, with the method including assembling a perforating string and, in a single trip of the perforating string into the wellbore: a) perforating the wellbore, b) fracturing the formation by increasing pressure in the wellbore, and c) decreasing pressure in the wellbore by exposing the wellbore to a low pressure volume.

In a further aspect, a method can include igniting a propellant in a wellbore, thereby increasing pressure in the wellbore and fracturing the formation, and then decreasing pressure in the wellbore by exposing the wellbore to a low pressure volume. The wellbore is not necessarily perforated in this method.

A still further aspect involves a method of enhancing communication between a wellbore and an earth formation having an initial formation pressure, with the method including increasing pressure in the wellbore and thereby fracturing the formation, and then, while the wellbore pressure is greater than the formation pressure, decreasing pressure in the wellbore by exposing the wellbore to a low pressure volume.

A method of enhancing communication between a wellbore and an earth formation intersected by the wellbore can include forming a fracture in the formation, and permitting communication between the wellbore and a low pressure volume in response to a desired characteristic of the fracture being maximized.

Another method of enhancing communication between a wellbore and an earth formation can include forming a fracture in the formation; and then, while pressure in the formation proximate the wellbore is at least about a closure pressure of the fracture, decreasing pressure in the wellbore by exposing the wellbore to a low pressure volume.

These and other features, advantages and benefits will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative examples below and the accompanying drawings, in which similar elements are indicated in the various figures using the same reference numbers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic partially cross-sectional view of a well system and associated method which can embody principles of the present disclosure.

FIG. 2 is an enlarged scale schematic cross-sectional view of perforations extending into an earth formation in the system and method of FIG. 1.

FIG. 3 is a schematic partially cross-sectional view of a delay device which may be used in the system and method of FIG. 1.

FIG. 4 is a schematic partially cross-sectional view of a valve which may be used in the system and method of FIG. 1.

FIG. 5 is a schematic flow chart for the method.

DETAILED DESCRIPTION

Representatively illustrated in FIG. 1 is a well system 10 and associated method which can embody principles of this disclosure. Of course, the scope of this disclosure is not limited only to the specific examples of the well system 10 and method described herein, since other well systems and methods can be used, if desired.

In the example depicted in FIG. 1, a perforating string 12 is positioned in a section of a wellbore 14 which intersects an earth formation 16. The perforating string 12 includes a perforator 18, a delay device 20 and a valve 22.

The perforator 18 is illustrated in FIG. 1 as a tubing conveyed perforating gun (e.g., having perforating charges contained within a tubular gun housing, etc.). However, other types of perforators (such as, wireline-conveyed perforators, etc.) may be used instead.

A firing head 24 may be used to control firing of the perforator 18. Any type of firing head (mechanical, electrical, telemetry controlled, etc.) which is compatible with the perforator 18 can be used.

A propellant 26 is associated with the perforator 18, so that the propellant is ignited in response to the perforator being fired. One suitable propellant sleeve is included with a perforating gun and marketed as the STIMGUN™ by Halliburton Energy Services, Inc. of Houston, Tex. USA.

The STIMGUN™ could be used for the perforator 18 and propellant 26 in the perforating string 12. However, other types of propellants and perforators can be used instead, in keeping with the scope of this disclosure (for example, the propellant could be disposed inside the perforator, otherwise shaped or configured, etc.).

The delay device 20 provides a predetermined delay period between the perforator 18 firing and the valve 22 actuating. In an example described more fully below, the delay device 20 comprises a pyrotechnic fuse which is ignited when the perforator 18 is fired.

When the fuse is burned to its end, another propellant is ignited to cause the valve 22 to open. However, other types of delay devices 20, and other methods of actuating the valve 22 a predetermined delay period after firing the perforator 18, may be used in keeping with the scope of this disclosure.

The valve 22 controls fluid communication between the wellbore 14 and a low pressure volume 28. When the valve 22 is actuated, it exposes the wellbore 14 to the low pressure volume 28, thereby decreasing pressure in the wellbore.

The volume 28 is “low pressure” in that it is preferably substantially less than formation pressure in the formation 16 near the wellbore 14. The pressure in the volume 28 may be less than, greater than, or equal to, pressure in the wellbore 14 at the time the perforating string 12 is installed.

As depicted in FIG. 1, the volume 28 comprises an empty, sealed tubular housing having about atmospheric pressure therein. However, other types of low pressure volumes may be used instead, in keeping with the scope of this disclosure.

By appropriately sizing the volume 28 and adjusting the pressure therein, a desired pressure decrease in the wellbore 14 can be obtained in response to actuating the valve 22. A suitable valve for use in the perforating string 12 is the SURGEPRO™ marketed by Halliburton Energy Services, Inc., although other valves may be used and still remain within the scope of this disclosure.

SURGEPRO™ is also the name of a software package which can be used to determine a number of valves 22, and a number and size of the volume 28 to achieve a desired pressure decrease in the wellbore 14, and a corresponding pressure differential from the formation 16 to the wellbore. Another software package (PULSFRAC™, marketed by Halliburton Energy Services, Inc.) can be used to determine the amount of propellant 26 and the timing of the delay before the valve 22 is opened, to achieve the desired pressure differential from the formation 16 to the wellbore 14.

When the perforator 18 is fired, perforations 30 are formed extending outward from the wellbore 14, as schematically depicted in FIG. 2. As the propellant 26 burns, pressure is increased in the wellbore 14, which causes fluid flow (indicated by arrows 32 in FIG. 2) outward into the formation 16.

If the increased pressure in the wellbore 14 is sufficiently great, one or more fractures 34 may be formed in the formation 16, thereby providing for increased surface area exposure of the formation to the perforations 30. Eventually, the pressure in the wellbore 14 will maximize, and will no longer be great enough to cause fracturing of the formation 16.

At that point, it will be beneficial to substantially decrease pressure in the wellbore 14, so that fluid will flow back into the wellbore from the formation 16 (indicated by arrows 36 in FIG. 2), thereby backwashing, cleaning, scouring, and/or surging, etc., the fracture 34 and perforations 30. This will provide for enhanced fluid communication between the formation 16 and the wellbore 14 by removing perforating debris, scouring any compacted areas around the perforations 30, preparing the formation 16 for subsequent stimulation or conformance treatments, etc.

If the formation 16 is under-pressurized, low pressure, significantly depleted, etc., the system 10 can be used to provide sufficient differential pressure to accomplish the surging, cleaning, etc. discussed above. In the past, it was difficult with low pressure formations to achieve a sufficiently underbalanced condition in a wellbore to provide a large pressure differential to cause substantial inward flow from the formation to the wellbore.

However, in the system 10, the valve 22 is opened when the pressure in the wellbore 14 has been increased to greater than the initial formation pressure near the wellbore. In this manner, when pressure is decreased in the wellbore 14 (by exposing the wellbore to the low pressure volume 28), a relatively large pressure differential will be created to cause substantial fluid flow 36 from the formation 16 into the wellbore. Preferably, the valve 22 is opened just after the fracture 34 has ceased growing (thereby obtaining a maximum extent of the fracture into the formation 16), but other timings may be used within the scope of this disclosure.

The delay device 20 provides the predetermined delay period between firing the perforator 18 and opening the valve 22. It is contemplated that a delay period of a few hundred milliseconds would be appropriate, but other delay periods can be used, if desired.

Modeling software, such as the SURGEPRO™ system marketed by Halliburton Energy Services, Inc., may be used to determine optimum characteristics for the perforator 18, propellant 26, delay 20 and low pressure volume 28 to achieve desired perforation, stimulation and surging results. Of course, other means of designing the sizes, number, materials, etc., of the various components of the perforating string 12 may be used within the scope of this disclosure, and each operation would preferably be independently configured to achieve optimum results.

In one example, the SURGEPRO™ software is used in conjunction with another system marketed by Halliburton Energy Services, Inc. as the PULSFRAC™ system. In this example, the PULSFRAC™ software predicts near wellbore stimulation resulting from propellant ignition in a wellbore, and the SURGEPRO™ software predicts dynamic surges with modeling of perforating string volumes, surge valves, surge chambers, etc. The PULSFRAC™ and SURGEPRO™ software both incorporate models of the perforating string, wellbore geometry and formation characteristics.

The PULSFRAC™ software can be used to determine the amount of propellant needed to produce maximum fracturing without damage to downhole equipment. The PULSFRAC™ software can also plot fracture length, width, conductivity and skin, versus time.

For a given situation (e.g., a particular perforating string 12, wellbore 14, formation 16, etc.), the PULSFRAC™ software can indicate the amount of time from igniting the propellant 26 to the fracture 34 achieving a greatest characteristic (e.g., length, width, conductivity, minimized skin, etc.).

The SURGEPRO™ software can then be used to determine the optimum size of the low pressure volume 28 to achieve a desired pressure reduction in the wellbore 14 (e.g., for optimized surging of the formation 16 and cleaning of the perforations 30, etc.), with a delay between the perforator 18 being fired and the valve 22 being opened. This delay corresponds to the amount of time determined by the PULSFRAC™ software for the desired characteristic of the fracture 34 (e.g., length, width, conductivity, minimized skin, etc.) to reach its maximum level.

Thus, both stimulation and surging can be optimized, with the valve 22 being opened as the desired characteristic of the fracture 34 reaches its maximum level. In one example, the valve 22 can be opened when pressure in the formation 16 near the wellbore 14 is at or above a closure pressure of the fracture 34 (i.e., a pressure at which the fracture begins to close), and at or below a fracture pressure of the formation (i.e., a pressure at which fracturing begins in the formation).

This method can increase the pressure differential from the formation 16 to the wellbore 14 when the valve 22 is opened, as compared to conventional methods.

Referring additionally now to FIG. 3, an example of the delay device 20 is representatively illustrated, apart from the remainder of the perforating string 12. In this example, the delay device 20 includes a pyrotechnic fuse 38 and a booster charge 40 in an outer tubular housing 42.

At its upper end (as viewed in FIG. 3), the delay device 20 includes a connector 44 for sealingly connecting the delay device to the perforator 18. At its lower end, the delay device 20 includes another connector 46 for sealingly connecting the delay device to the valve 22.

When the perforator 18 is fired, the fuse 38 is ignited, and burns for a predetermined time. When the fuse 38 has burned to its lower end, the booster charge 40 is ignited, thereby actuating the valve 22.

A length of the fuse 38, a material of the fuse, etc., may be adjusted as needed to provide a desired delay period. Furthermore, other types of delay devices may be used within the scope of this disclosure.

Referring additionally now to FIG. 4, an example of the valve 22 is representatively illustrated apart from the remainder of the perforating string 12. The valve 22 depicted in FIG. 4 is similar to the SURGEPRO™ valve marketed by Halliburton Energy Services, Inc.

At its upper end (as viewed in FIG. 4), the valve 22 is provided with a connector 48 which connects the valve to the delay device 20. At its lower end (as viewed in FIG. 4), the valve 22 is provided with another connector 50 which connects the valve to the low pressure volume 28.

A slidable sleeve 52 is reciprocably disposed in a tubular housing 54 connected between the connectors 48, 50. Initially, the sleeve 52 blocks flow through the ports 56, thereby preventing fluid communication between the wellbore 14 and the low pressure volume 28.

However, when the booster charge 40 of the delay device 20 is ignited, a detonating cord 58 and propellant 60 in the valve 22 are ignited, thereby increasing pressure above the sleeve 52, forcing the sleeve to displace downward and open the ports 56. The resulting fluid communication between the wellbore 14 and the low pressure volume 28 decreases the pressure in the wellbore, as discussed above.

Referring additionally now to FIG. 5, a method 70 which can embody principles of this disclosure is representatively illustrated. In the method 70, the formation 16 is perforated (step 72), fractured or otherwise stimulated (step 74), and surged (step 76). Preferably, these steps 72-76 are performed in only a single trip of the perforating string 12 into the wellbore 14.

In step 72, the perforator 18 is fired, thereby perforating the wellbore 14 and forming the perforations 30 extending outward into the formation 16. This achieves a level of fluid communication between the wellbore 14 and the formation 16.

If the perforations 30 already exist, then step 72 may not include perforating the wellbore 14. Instead, the propellant 26 could be ignited without firing the perforator 18.

In step 74, fluid 32 flows into the formation 16 from the wellbore 14 due to increased pressure in the wellbore. This increased pressure may be due to igniting of the propellant 26 in response to firing the perforator 18 (or igniting the propellant without firing the perforator). The increased pressure in the wellbore 14 is preferably greater than a fracture pressure of the formation 16, so that one or more fractures 34 are formed in the formation.

In step 76, pressure in the wellbore 14 is substantially decreased, for example, by opening the valve 22 to expose the wellbore to the low pressure volume 28. Pressure may be decreased in the wellbore 14 just after (or as) the fracture(s) 34 have reached their maximum extent in the formation 16.

The decreased pressure in the wellbore 14 causes fluid flow 36 back into the wellbore, thereby cleaning, scouring, etc., the fracture(s) 34 and perforations 30. Preferably, but not necessarily, the decreased pressure in the wellbore 14 can be less than the initial formation pressure in the formation 16 near the wellbore.

Although the SURGEPRO™ and PULSFRAC™ software packages are mentioned above as being used to configure the components of the perforating string 12 (e.g., the number of valves 22 and low pressure volumes 28, the size of the low pressure volume, the delay between propellant 26 igniting and valve opening, etc.), it should be clearly understood that other software packages, and other means of determining the various characteristics of the perforating string, may be used in keeping with the scope of this disclosure.

Although certain examples described above use the delay device 20 in order to open the valve 22 as the fracture 34 characteristic is maximized, in other examples it may be desired to use other timings. For example, it could be advantageous to reduce the delay, in order to increase the pressure differential from the formation 16 to the wellbore 14 and thereby optimize surging, at the expense of a less than optimum fracture characteristic.

It may now be fully appreciated that this disclosure provides several advancements to the art of enhancing communication between a wellbore and a formation intersected by the wellbore. In the examples described above, stimulating and surging can be performed with a single trip into a wellbore. The wellbore can also be perforated in the single trip into the wellbore. Furthermore, low pressure formations can be adequately surged after fracturing.

The above disclosure provides to the art a method 70 of enhancing communication between a wellbore 14 and an earth formation 16 intersected by the wellbore 14, the formation 16 having a formation pressure. The method 70 comprises: fracturing the formation 16 in response to firing a perforator 18 in the wellbore 14; and then, a predetermined delay period after firing the perforator 18, decreasing pressure in the wellbore 14. The pressure in the wellbore 14 may be decreased to less than the formation pressure.

Decreasing pressure can be initiated while the pressure in the wellbore 14 is greater than the formation pressure.

A delay device 20 preferably causes the predetermined delay period after firing the perforator 18. The delay device 20 may include an ignitable fuse 38.

Decreasing pressure in the wellbore 14 can include opening a valve 22. Opening the valve 22 can include permitting fluid communication between the wellbore 14 and a low pressure volume 28.

The method 70 can include increasing pressure in the wellbore 14 prior to decreasing pressure in the wellbore 14. Increasing pressure in the wellbore 14 can include igniting a propellant 26 in response to firing the perforator 18.

Increasing pressure in the wellbore 14 can include fracturing the formation 16. Decreasing pressure in the wellbore 14 may be performed after fracturing the formation 16.

A well system 10 provided by the above disclosure can comprise a perforating string 12 positioned in a wellbore 14, the perforating string 12 including a perforator 18, a delay device 20 and a valve 22. The valve 22 actuates and thereby decreases pressure in the wellbore 14 a predetermined delay period after the perforator 18 is fired.

In response to the perforator 18 being fired, a propellant 26 may be ignited, which increases pressure in the wellbore 14. The valve 22 may actuate after the increased wellbore pressure fractures a formation 16. The valve 22 may actuate while the increased wellbore pressure is greater than formation pressure.

The delay device 20 can delay actuation of the valve 22 after the perforator 18 is fired.

Also described above is a method 70 of enhancing communication between a wellbore 14 and an earth formation 16, with the method 70 comprising: assembling a perforating string 12; and in a single trip of the perforating string into the wellbore 14: a) perforating the wellbore 14, b) fracturing the formation 16 by increasing pressure in the wellbore 14, and c) decreasing pressure in the wellbore 14 by exposing the wellbore to a low pressure volume 28.

Increasing pressure in the wellbore 14 can include igniting a propellant 26 in response to perforating the wellbore 14. Increasing pressure in the wellbore 14 may include increasing the wellbore pressure to greater than the formation pressure.

Decreasing pressure in the wellbore 14 may be initiated while the wellbore pressure is greater than the formation pressure. Decreasing pressure in the wellbore 14 can include opening a valve 22 a predetermined delay period after perforating the wellbore 14.

The above disclosure also describes a method 70 of enhancing communication between a wellbore 14 and an earth formation 16 having an initial formation pressure, in which the method 70 includes igniting a propellant 26 in the wellbore 14, thereby increasing pressure in the wellbore 14 and fracturing the formation 16, and then decreasing pressure in the wellbore 14 by exposing the wellbore 14 to a low pressure volume 28.

The method 70 may include assembling a perforating string 12, and performing the propellant 26 igniting and pressure decreasing steps in a single trip of the perforating string 12 into the wellbore 14. Increasing pressure in the wellbore 14 can include igniting the propellant 26 in response to perforating the wellbore 14.

Increasing pressure in the wellbore 14 may include increasing the wellbore 14 pressure to greater than the formation 16 pressure. Decreasing pressure in the wellbore 14 may be initiated while the wellbore 14 pressure is greater than the formation 16 pressure.

Decreasing pressure in the wellbore 14 may include opening a valve 22 a predetermined delay period after increasing pressure in the wellbore 14.

A method 70 of enhancing communication between a wellbore 14 and an earth formation 16 intersected by the wellbore 14 can include forming a fracture 34 in the formation 16; and permitting communication between the wellbore 14 and a low pressure volume 28 in response to a desired characteristic of the fracture 34 being maximized.

The desired characteristic may be chosen from a group comprising length, width, conductivity, and reduced skin.

Permitting communication can be performed while pressure in the formation 16 proximate the wellbore 14 is at least about a closure pressure of the fracture.

Permitting communication can further be performed while the pressure in the formation 16 proximate the wellbore 14 is no more than about a fracture pressure of the formation 16.

Permitting communication can include further comprises decreasing pressure in the wellbore 14 to less than a formation pressure of the formation 16.

Permitting communication may be initiated while pressure in the wellbore 14 is greater than a formation pressure of the formation 16.

A delay device 20 may produce a predetermined delay period between initiating the fracture 34 and permitting communication between the wellbore 14 and the low pressure volume 28. The delay device 20 may comprise an ignitable fuse 38.

Permitting communication can comprise opening a valve 22.

Forming the fracture 34 can include increasing pressure in the wellbore 14 prior to permitting communication between the wellbore 14 and the low pressure volume 28.

Forming the fracture 34 can be performed in response to igniting a propellant 26 in the wellbore 14. Igniting the propellant 26 may be performed in response to firing a perforator 18.

The method can also include assembling a perforating string 12; and perforating the wellbore 14, and performing the fracture 34 forming and communication permitting steps, in a single trip of the perforating string 12 into the wellbore 14.

A method 70 of enhancing communication between a wellbore 14 and an earth formation 16 can include forming a fracture 34 in the formation 16; and then, while pressure in the formation 16 proximate the wellbore 14 is at least about a closure pressure of the fracture 34, decreasing pressure in the wellbore 14 by exposing the wellbore 14 to a low pressure volume 28.

Decreasing pressure in the wellbore 14 can be performed while the pressure in the formation 16 proximate the wellbore 14 is no greater than a fracture pressure of the formation 16.

Decreasing pressure in the wellbore 14 may be performed in response to a desired characteristic of the fracture 34 being maximized. The desired characteristic may be chosen from a group comprising length, width, conductivity, and reduced skin.

Decreasing pressure can include decreasing pressure in the wellbore 14 to less than a formation pressure of the formation 16.

Decreasing pressure in the wellbore 14 may be initiated while the pressure in the wellbore 14 is greater than a formation pressure of the formation 16.

A delay device 20 may produce a predetermined delay period between initiating the fracture 34 and decreasing pressure in the wellbore 16. The delay device 20 may comprise an ignitable fuse 38.

Decreasing pressure in the wellbore 14 may comprise opening a valve 22.

The method 70 may also include: assembling a perforating string 12; and perforating the wellbore 14, and performing the fracture 34 forming and pressure decreasing steps, in a single trip of the perforating string 12 into the wellbore 14.

It is to be understood that the various examples described above may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present disclosure. The embodiments illustrated in the drawings are depicted and described merely as examples of useful applications of the principles of the disclosure, which are not limited to any specific details of these embodiments.

In the above description of the representative examples of the disclosure, directional terms, such as “above,” “below,” “upper,” “lower,” etc., are used for convenience in referring to the accompanying drawings.

Of course, a person skilled in the art would, upon a careful consideration of the above description of representative embodiments, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to these specific embodiments, and such changes are within the scope of the principles of the present disclosure. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims and their equivalents. 

1. A method of enhancing communication between a wellbore and an earth formation intersected by the wellbore, the method comprising: forming a fracture in the formation; and permitting communication between the wellbore and a low pressure volume in response to a desired characteristic of the fracture being maximized.
 2. The method of claim 1, wherein the desired characteristic is chosen from a group comprising length, width, conductivity, and reduced skin.
 3. The method of claim 1, wherein permitting communication is performed while pressure in the formation proximate the wellbore is at least about a closure pressure of the fracture.
 4. The method of claim 3, wherein permitting communication is further performed while the pressure in the formation proximate the wellbore is no more than about a fracture pressure of the formation.
 5. The method of claim 1, wherein permitting communication further comprises decreasing pressure in the wellbore to less than a formation pressure of the formation.
 6. The method of claim 1, wherein permitting communication is initiated while pressure in the wellbore is greater than a formation pressure of the formation.
 7. The method of claim 1, wherein a delay device produces a predetermined delay period between initiating the fracture and permitting communication.
 8. The method of claim 7, wherein the delay device comprises an ignitable fuse.
 9. The method of claim 1, wherein permitting communication further comprises opening a valve.
 10. The method of claim 1, wherein forming the fracture further comprises increasing pressure in the wellbore prior to permitting communication.
 11. The method of claim 1, wherein forming the fracture is performed in response to igniting a propellant in the wellbore.
 12. The method of claim 11, wherein igniting the propellant is performed in response to firing a perforator.
 13. The method of claim 1, further comprising: assembling a perforating string; and perforating the wellbore, and performing the fracture forming and communication permitting steps in a single trip of the perforating string into the wellbore.
 14. A method of enhancing communication between a wellbore and an earth formation, the method comprising: forming a fracture in the formation; and then, while pressure in the formation proximate the wellbore is at least about a closure pressure of the fracture, decreasing pressure in the wellbore by exposing the wellbore to a low pressure volume.
 15. The method of claim 14, wherein decreasing pressure in the wellbore is performed while the pressure in the formation proximate the wellbore is no greater than a fracture pressure of the formation.
 16. The method of claim 14, wherein decreasing pressure in the wellbore is performed in response to a desired characteristic of the fracture being maximized.
 17. The method of claim 16, wherein the desired characteristic is chosen from a group comprising length, width, conductivity, and reduced skin.
 18. The method of claim 14, wherein decreasing pressure further comprises decreasing pressure in the wellbore to less than a formation pressure of the formation.
 19. The method of claim 14, wherein decreasing pressure in the wellbore is initiated while the pressure in the wellbore is greater than a formation pressure of the formation.
 20. The method of claim 14, wherein a delay device produces a predetermined delay period between initiating the fracture and decreasing pressure in the wellbore.
 21. The method of claim 20, wherein the delay device comprises an ignitable fuse.
 22. The method of claim 14, wherein decreasing pressure in the wellbore further comprises opening a valve.
 23. The method of claim 14, wherein forming the fracture is performed in response to igniting a propellant in the wellbore.
 24. The method of claim 23, wherein igniting the propellant is performed in response to firing a perforator.
 25. The method of claim 14, further comprising: assembling a perforating string; and perforating the wellbore, and performing the fracture forming and pressure decreasing steps, in a single trip of the perforating string into the wellbore. 